(1) Although bottle A is open-air, the bacteria only fall on the bend of the goose neck bottle, and the broth is still fresh. B Bottle neck is straight, and bacteria in the air enter the broth and multiply, which makes the broth deteriorate. It can be seen that the function of the bent glass tube in the "Goose Neck Bottle" is that air can pass through, but bacteria can't.

(2) According to (1), the conditions of bottle A and bottle B should be the same except whether there is bacteria. So the variable in the experiment is bacteria, and the bacteria in the air can enter the bottle B as the experimental group, but not the bottle A as the control group.

(3) The purpose of boiling the broth before the experiment is to kill the bacteria in the broth, eliminate the interference of non-experimental factors in the broth itself, and make the experimental results credible and convincing.

(4) Although the bottle A is open, the bacteria only fall on the bend of the goose neck bottle, and the broth is still fresh; After the bottleneck is broken, the bacteria in the air enter the broth and multiply in large numbers, which makes the broth deteriorate.

(5) Dilution-coating plate method is to dilute the bacterial liquid into a series of gradients, and then coat the bacterial liquid with different dilution on the surface of agar solid medium for culture. In the bacterial liquid with high enough dilution, the aggregated microorganisms will disperse into single cells, thus forming a single colony on the surface of the culture medium. When a small amount of Escherichia coli was inoculated in LB liquid culture medium, turbidity appeared in the culture medium, which indicated that Escherichia coli had proliferated in large quantities.

(6) It is a special sampling method to estimate microbial density with blood cell counting plate, which needs to count yeasts in cells, adjacent edges and corners; By this method, it is found that the density of B is about 2 times that of A, and then multiplied by the dilution multiple (100 times), so the density of B is 200 times that of A?

(7) The number of microorganisms in a container can be measured by using the blood cell counting board, and then the number of bacteria (including dead cells) can be calculated.

The answer is: (1) fresh (not deteriorating); Deterioration (bad taste); Air can pass through, but bacteria can't;

(2) the existence of bacteria;

(3) Kill the bacteria in the broth and eliminate the bacteria in the broth itself (sterilization);

(4) breaking through the bottleneck;

(5) turbidity; Dilution coating flat plate (or dilution coating);

(6)200;

(7)D。